Gastrulation is a process where the provisional spatial organization of the egg is transformed into the definitive body plan of the adult. In vertebrate organisms, the major developmental mechanism for establishing the body plan during gastrulation is the Spemann Organizer. The Organizer has no obvious parallels in the major invertebrate phyla, although it is present in invertebrate Chordates, like Ascidians. The major features of the Organizer in gastrulation are the specification of cell behaviors, required to create the adult body plan, the existence of several key signaling systems (whose origins in several cases predate the vertebrate emergence), and the coordinated secretion of proteins that regulate and, in particular, inhibit several pathways. We have taken three ambitious well defined approaches to gastrulation. In the first, we try to understand how the gene brachyury controls the decision of whether cells migrate, as they do in head mesoderm or undergoes a multicellular sorting behavior called convergent extension, in the chordamesoderm. We plan to examine genes downstream of brachyury that control adhesion to fibronectin and cell motility. As part of these studies, we will take a closer look at the mechanism of cell polarization in multicellular populations by confocal microscopy and, in particular, the role of microtubules in polarization. In the second, we try to understand how the Wnt signaling pathway interprets and processes signals, by reconstituting large portions of it in cell free extracts from Xenopus eggs. We will look particularly carefully at the role of Casein Kinase 1 and the mechanism of Axin degradation. We will also look at another important pathway, the MAP kinase pathway as to how it processes signals, making quantitative measurements of rates and fluxes and modeling it mathematically. Finally, we will step as far back as we can from vertebrate gastrulation to a sister phylum of Chordates, Hemichordates, to examine whether there are features of the Organizer that can be identified that may lead to an understanding of how gastrulation in Chordates became the major mechanism for organizing the body plan. Our major approach here is to examine the early embryology of the acorn worm, Saccoglossus kowalevskii, and identify genes which in vertebrates play a major role in the Organizer.